This invention concerns improved siliceous packings having chemically bonded polysiloxane coatings for use in gas chromatographic analysis and a process for making these packings. Chromatographic apparatus based on these bonded packings and the analytical methods thereby made possible are also parts of the invention.
The field of chromatographic analysis has expanded rapidly in recent years and many refinements in apparatus, in analytical methods, and in the packings for chromatographic columns have been developed in response to the ever-increasing demand for faster and more sophisticated analyses and the need for differentiating and identifying extremely small concentrations of compounds, some of which may be structurally closely related. As a result of these pressures, many modifications of chromatographic column packing materials have been made to improve their properties. Gas chromatography in which a mixture of a carrier gas and the vapor of a substance to be separated into its individual component compounds is contacted with a chromatographic packing has become increasingly useful. Siliceous supports such as glass beads, silica beads, silica gel, and diatomaceous earth have been physically coated or coated by chemical bonding with various organic materials to deactivate the active surface of the support and to provide more efficient use of the desirable chromatographic properties of the coating material itself.
Aue et al., J. Chromatogr. 42, 319 (1969), Majors et al., J. Chromatogr. Sci. 12, 767 (1974), and Grushka et al., Anal. Chem. 49, 1005A (1977) are representative of many publications in this area of investigation which describe the preparation for chromatographic use of siliceous column packings having siloxane polymer molecules chemically bonded to their surface. These preparations typically are based on the reaction of an acid-activated silica surface with an organopolychlorosilane in solvent solution with water present to cause hydrolysis and polymerization. The terminal hydroxyl groups on the polymer molecules thereby chemically bonded to the silica surface are then inactivated by a capping reaction with trimethylchlorosilane or similar silicon compound. Alternatively, terminal chlorosilyl groups can be inactivated by reaction with an alcohol such as methanol to produce a terminal ether substituent. The bonded products have non-extractable siloxane polymer coatings of up to 5-10 percent by weight and are useful as column packings for chromatographic use. However, these relatively heavy coatings of polymer prepared in this way do not coat the silica surface uniformly and the coated supports do not have the optimum chromatographic properties which are particularly required in gas chromatography.
An improved method has now been discovered by which an active silica or other active siliceous surface can be coated uniformly by a relatively very thin layer of organosiloxane polymer molecules bonded chemically to the surface and covering the surface so as to negate surface activity essentially completely. The bonded product has properties useful in gas chromatography that were not obtainable by previously known methods of preparation.